BEACON Researchers at Work: What makes invasive species successful?

This week’s BEACON Researchers at Work blog post is by MSU graduate student Amanda Charbonneau.

Amanda Charbonneau next to one of her tallest plants – she’s 5’6″!

I love to walk through the woods on a quiet quest to see how many woodland creatures I can spot, and to take an inventory of what’s new and blooming. If you spend enough time exploring same woodlot, you begin to notice when new organisms start creeping in. Throughout Michigan, prairies are filling with Autumn Olive, an invasive species from Asia that most of us know only as the silvery green bush growing alongside the expressway. Similarly, some of my favorite woodland paths are now nearly impassable mats of multiflora rose, a thorny Asian species once planted as living fences. The Michigan DNR estimates that there are about 200 invasive plant and animal species in Michigan, most of which accidently (or occasionally purposefully) established here in the last few hundred years.

Two hundred may seem like a large number of organisms, but they aren’t the only non-native species to have found themselves in Michigan over the years. For every stowaway Emerald Ash Borer that successfully establishes itself and becomes a major concern, there must be dozens of other burrowing insects that got here the same way, but didn’t become invasive. There are gardens full of exotic plants in every neighborhood, and yet only a handful, like Garlic Mustard, have escaped to become a pest. There are more than a million catalogued plant and animal species on earth, and yet the number that acts like invasive species is relatively small. One estimate, called the ten’s rule, is that for every thousand species that disperses out of it’s native range, only 100 will survive the dispersal, only 10 of those will establish in a new range, and only one of those will successfully reproduce and become invasive.

So why aren’t all species invasive when given the chance? Or to state it another way: Why are invasive species able to survive in new environments, when most other organisms can’t?

A page from The Herball of Generall Historie of Plantes, by John Norton (1957) – one of the earliest references to weedy radish.

My research is to determine how potentially weedy species adapt to new environments. It may sound a bit odd to try to learn about invasive species by looking at a weed, but weeds are a good model system for studying invasive species because they tend to invade the places that we care about the most: our yards, gardens and agricultural fields. This makes them disproportionately costly, and the US more than 34 billion dollars a year on weed management. I specifically work on the plants in the genus Raphanus which includes crop radishes, weedy radish, and a number of wild radish plants. Weedy radish, tend to be a problem mostly in wheat, barley and oat fields, where they crowd out desirable crops and contaminate the harvested grains.

One of the coolest things about weedy radish is that they have two close relatives: crop radishes and wild radish. This means that I can compare the physical and genetic characteristics of all three to try to learn more about how the weed evolved. For instance, the weedy and crop radishes grow in fields all over the world, but wild radish only grows around the Mediterranean and mostly in marginal places like beaches, so even though they are closely related, they live in very different environments.

One-month-old radish plants, wild (top) and weedy (bottom)

Another really interesting way these plants differ is in their growth rate. Farmers only grow wheat for 3 or 4 months before it’s harvested, and everything else gets tilled under, so in order to survive in a field, you have to grow very fast. Weedy radish can go from germination to flowering and starting to produce seed in as little as 30 days, so they can easily reproduce in that time frame. However the wild plants often take more than 100 days to start flowering, and some populations need to grow for an entire year before they can make seeds. This is important, because it suggests that fast growth is a trait that is under intense selection. When wild radish first moved into wheat fields, nearly all of the plants would have gotten tilled under every year without reproducing. However, a very fast growing one might make a few seeds, which would be better able to survive the following season. Since this is an adaptation to tilling, this trait must have evolved since humans started farming.

The difference in growth rate is impressive, but could have just been due to where they were grown. There are, after all, lots of differences between Mediterranean beaches and wheat fields in Kalamazoo, MI. To verify that the differences between the weedy and wild wheat were due to genetics and not environment, I’ve done three common garden experiments with hundreds of plants each. In these experiments, I grew weeds taken from all over the world as well as several wild populations and some crops all in the same large field. However, instead of setting up the plants in orderly groups like you might in your garden, I chose each individual plants’ location randomly. This arrangement tends to drown out all of the small differences in environment across the field, so that all of the differences you see in physical characteristics are based on genetics. In these experiments, there are always dramatic differences between how long it takes the weedy and wild radish to flower.

Now that we’re sure the differences between wild and weedy radish are genetic I’m sequencing several weeds and wild plants to find the places where they differ genetically. Since we know all of the plants are closely related, we expect that most of their genes will be very similar, and the few differences we see in their genomes should correlate with their physical differences. Once I have all the sequencing results back, I should be able to find things like the genes that allow weedy radish to grow so much faster than the wild version, or genes that allow weedy radish to flourish in fields instead of beaches.

If we can find the genomic regions that control things like growth rate, that’s a trait that crop breeders might be interested in exploiting. They might also be give us a place to start looking for important genomic regions in other weeds, and maybe even more typical invasive plants, since fast growth is one of the commonalities many of them share. From an evolutionary point of view, it’s also important just to understand how weeds came about. New weeds and invasive species are evolving all the time, and the more we know about how they occur, the better our chances of slowing them down when they next show up on our doorstep.

For more information about Amanda’s work, you can contact her at charbo24 at msu dot edu.